﻿of 
  Films 
  of 
  Cobalt 
  fyc. 
  in 
  Magnetic 
  Fields. 
  245 
  

  

  The 
  film 
  formed 
  one 
  arm 
  of 
  a 
  Wheatstone 
  bridge 
  with 
  

   resistance-boxes 
  in 
  its 
  other 
  arms, 
  and 
  in 
  addition 
  a 
  graduated 
  

   german-silver 
  wire, 
  PQ, 
  with 
  a 
  movable 
  contact-maker. 
  The 
  

   current 
  was 
  supplied 
  by 
  the 
  battery 
  of 
  two 
  secondary 
  cells 
  

   already 
  referred 
  to 
  ; 
  this 
  current 
  had 
  to 
  be 
  very 
  weak 
  — 
  ■ 
  

   usually 
  a 
  few 
  milliamperes 
  — 
  in 
  order 
  to 
  avoid 
  heating. 
  

  

  The 
  resistances 
  of 
  the 
  four 
  arms 
  of 
  the 
  bridge 
  were 
  made 
  

   as 
  nearly 
  equal 
  as 
  possible. 
  

  

  When 
  a 
  balance 
  had 
  been 
  obtained 
  in 
  the 
  Wheatstone 
  

   bridge, 
  the 
  magnetic 
  field 
  was 
  created 
  ; 
  the 
  balance 
  previously 
  

   existing 
  was 
  disturbed, 
  that 
  is, 
  a 
  defleKion 
  w 
  v 
  as 
  now 
  observed 
  

   on 
  the 
  galvanometer 
  due 
  to 
  the 
  fact 
  that 
  the 
  resistance 
  of 
  

   the 
  film 
  had 
  chanoed. 
  This 
  reading 
  was 
  then 
  assumed 
  

   proportional 
  to 
  the 
  current 
  flowing 
  through 
  the 
  galvanometer, 
  

   that 
  is 
  proportional 
  to 
  Aa, 
  the 
  variation 
  of 
  the 
  resistance, 
  a, 
  

   of 
  the 
  film. 
  Such 
  an 
  assumption 
  gives 
  a 
  result 
  less 
  than 
  the 
  

   true 
  one 
  by 
  0*003 
  of 
  the 
  total 
  result 
  in 
  the 
  most 
  unfavourable 
  

   case 
  ; 
  in 
  other 
  words, 
  with 
  a 
  reading 
  of 
  300 
  mm. 
  the 
  error 
  

   is 
  1 
  mm. 
  

  

  The 
  steady 
  magnetic 
  field 
  was 
  created 
  by 
  means 
  of 
  a 
  small 
  

   Ruhmkorff 
  electromagnet 
  with 
  pole-pieces 
  10 
  mm. 
  in 
  diameter 
  

   and 
  between 
  3 
  and 
  4 
  mm. 
  apart. 
  The 
  electromagnet 
  with 
  

   its 
  pole-pieces 
  was 
  pierced 
  through 
  the 
  centre 
  so 
  that 
  the 
  

   field-strength 
  could 
  be 
  determined 
  by 
  the 
  rotation 
  of 
  the 
  

   plane 
  of 
  polarization 
  in 
  a 
  piece 
  of 
  glass. 
  

  

  The 
  galvanometer 
  was 
  placed 
  in 
  such 
  a 
  position 
  as 
  to 
  be 
  

   uninfluenced 
  by 
  the 
  magnet. 
  It 
  had 
  a 
  resistance 
  of 
  10 
  ohms. 
  

   Its 
  scale 
  was 
  placed 
  about 
  3 
  metres 
  distant 
  from 
  its 
  mirror. 
  

  

  The 
  Hall 
  effect 
  is 
  given 
  in 
  the 
  results 
  only 
  in 
  scale- 
  

   readings. 
  The 
  variation 
  of 
  resistance 
  is 
  given 
  in 
  scale-readings 
  

   and 
  in 
  absolute 
  measure. 
  

  

  To 
  find 
  what 
  variation 
  of 
  resistance 
  in 
  the 
  film 
  was 
  

   indicated 
  by 
  a 
  galvanometer 
  reading 
  of 
  one 
  scale-division, 
  

   the 
  following 
  method 
  was 
  employed. 
  A 
  suitable 
  resistance 
  — 
  

   usually 
  100,000 
  ohms 
  — 
  was 
  placed 
  in 
  parallel 
  with 
  an 
  arm 
  

   of 
  the 
  bridge 
  ; 
  the 
  point 
  on 
  the 
  graduated 
  scale 
  which 
  was 
  

   such 
  as 
  to 
  give 
  no 
  current 
  through 
  the 
  galvanometer 
  was 
  

   found 
  when 
  the 
  parallel 
  circuit 
  of 
  100,000 
  ohms 
  was 
  broken; 
  

   this 
  latter 
  was 
  then 
  made 
  and 
  the 
  reading 
  observed. 
  Evi- 
  

   dently, 
  knowing 
  the 
  resistance 
  of 
  the 
  arm 
  of 
  the 
  bridge 
  

   concerned, 
  of 
  the 
  shunt, 
  and 
  knowing 
  the 
  deflexion 
  produced, 
  

   the 
  variation 
  of 
  resistance 
  represented 
  by 
  one 
  scale-division 
  

   deflexion 
  on 
  the 
  galvanometer 
  could 
  be 
  calculated. 
  

  

  A 
  plus 
  sign 
  denotes 
  an 
  increase, 
  a 
  minus 
  sign 
  a 
  decrease 
  

   of 
  resistance. 
  

  

  In 
  most 
  of 
  the 
  films 
  the 
  Hall 
  effect 
  was 
  observed 
  first 
  ; 
  

  

  